A transition from a microstrip to a closed waveguide is a key component in microwave technology.
The current high volume trend in electronics and microwave designs is to use traditional circuit board techniques for the integration of packaged microwave circuits, and it is thus desirable to make transitions from microstrip to closed waveguide with a design that allows for the use of so called surface mount technology, usually abbreviated as SMT.
One popular design for such transitions is the so called E-probe, which comprises a closed waveguide with a pin probe which protrudes from one of the closed waveguide's walls into the closed waveguide roughly a quarter of a wave length from the closed waveguide's end. Although such a transition is not based on SMT-components, it allows the use of traditional SMT-boards.
Another alternative is to let a microstrip to closed waveguide transition be based on a so called ridge waveguide. In this case, there is first a transition from microstrip to ridge wave guide, and then a transition from ridge waveguide to closed waveguide. Electromagnetic propagation takes place along the circuit board and along the microstrip. Such a solution provides SMT compatibility.
Some drawbacks with these known technologies are as follows: An E-probe transition gives high loss since the electromagnetic field has to travel through a dielectric material on the circuit board. Due to band width limitations in combination with variations in etching, inner-layer registration, positions of vias, etc, it becomes increasingly difficult to use this technology with increasing frequencies and/or bandwidth. Another drawback with an E-probe transition is that it requires two waveguide pieces, one on each side of the board.
A transition based on a ridge waveguide will have electromagnetic leaks around the ridge waveguide's end. In most cases, the transition is arranged inside a metallic enclosure, which will create electromagnetic resonances unless the enclosure is filled with absorbing material. Another drawback of a transition based on a ridge waveguide is that reliable galvanic contact must be made where the microstrip meets the ridge. A certain size of such a joint is also required in order to enable reliable contact, which leads to limited design freedom in the microwave optimization, which in turn limits the bandwidth of the transition.